The invention relates to a photometer using the non-dispersive infrared spectroscopy method, NDIR method for short, having a measuring cell, an infrared radiator with radiator modulation, the measuring cell consisting of a measurement and comparison chamber, and having at least one optopneumatic detector, in accordance with the preamble of Patent Claim 1.
The output characteristic of absorption photometers, which also include the NDIR photometer, obeys the Lambert-Beer law. The desired linear relationship between concentration and output current requires electronic measures forlinearization. In addition to pure absorption, however, there is also extinction along the beam path through the cell. To this extent, the measuring range is limited in general by a maximum product of cell length and concentration. Here, pure extinction is the non-selective general attenuation of radiation by gases or solids. Extinction too effects attenuation of the original signal and therefore simulates absorption. To this extent, the cell lengths cannot be selected arbitrarily.
Moreover, optopneumatic photometers are known whose gas-filled detectors can be connected in series, with the result that it is possible to determine two or more components simultaneously with only one measuring cell. However, this procedure fails in the case of very different measuring ranges, because of the abovementioned problem with the output characteristic. This is the case, for example, with the more or less routine analysis of combustion gases. The aim here is frequently to determine the generally low CO concentration (100 ppm) and the high CO2 concentration (15% by volume). It is usually necessary to construct two beam paths with different cell lengths in order to solve this problem.
Furthermore, there is known from DE 44 19 458 a method for measuring the purity of CO2 in which the measurement of natural CO2 is limited to the absorption band f the isotope 13CO2. In this case, the method is tuned and configured exclusively such that only purity measurements of this one measuring component can be carried out.
The problem is thus that it is mostly necessary to use a plurality of measuring cells in the case of a desired measurement of a plurality of components, since the at least two components to be measured occur in different measuring ranges. One example of this is the already known method of using NDIR spectroscopy to measure the ratio of 12CO2 and 13CO2 in separate beam paths. Since the concentrations to be measured differ by approximately 1:100, each channel is provided with a dedicated measuring cell differing in length. The different lengths are selected in order to render it possible to linearize the output characteristics, which sag in accordance with the Lambert-Beer law. Both cells are charged in parallel with measuring gas. As a result of the measurement, the individual components, and also the quotient of 13CO2/12CO2 are output. Owing to the fact that the cells can be charged with measuring gas in a fashion which is not exactly simultaneous, when a quotient is formed during an online measurement of, for example, respiratory air unavoidable dynamic errors occur which cause large dynamic deviations when conducting online measurement of a proband. Added to this, again, is the overall problem, already outlined, of handling isotope ratios of an at least chemically identical gas component.
Moreover, the two measurement results must, again, be further computed in order to obtain some degree of mutual correspondence, since it is, after all, one and the same measuring gas which is concerned, even if it consists of a plurality of components. The reason why this is so critical is that the NDIR method is an absorption method. That is to say, the higher the concentration in the measuring gas of the component actually to be measured, the higher the specific absorption inside the gas. That is to say, given high concentrations all that remains is a small residual signal which reaches the detector. The remaining radiation intensity is, however, decisive, again, for producing the measuring effect, since the detectors depend in this gas-filled form on the optopneumatic effect. That is to say, with ever larger concentrations the residual signal paradoxically becomes ever smaller, and thus also ever less accurate. By contrast, low concentrations can be measured accurately because owing to the low concentration the specific absorption inside the measuring cell is also correspondingly low, with the result that a relatively high light signal remains to excite the detector. This problem, which essentially reflects the Lambert-Beer law is important in measuring a plurality of components.
It is therefore the object of the invention to render it possible to measure a plurality of components with high accuracy and the smallest possible outlay on apparatus.
According to the invention, the object adopted is achieved by means of the characterizing features of Patent Claim 1 in the case of an NDIR photometer of the generic type.
Further advantageous refinements of the invention are reproduced in the dependent claims.
The aim of this invention is to measure a plurality of components without the disadvantages of the measurement techniques of this type which are currently employed. By contrast therewith, it is to be possible for this measurement to be implemented simply and cost-effectively. The aim in this case is to use only a single measuring cell in order to achieve the same dynamic characteristic for the various measuring components. It is essential here to use a plurality of detectors which are connected in series and measure the individual gas components selectively. The gas components and/or the correspondingly selected absorption bands possible in this case must be selected here such that each detector exercises maximum absorption for the measuring component which it is to measure, and must be correspondingly transparent for the component which is to be detected in the downstream detector. Since the detectors enclose only relatively low gas volumes, the extinctions effected thereby from one detector to the other are negligible or at least known, and therefore capable of compensation.
The embodiment according to the invention assumes that the measuring component is present in its natural isotope abundance. Thus, it is known that natural CO2 consists of approximately 98.9% 12CO2 and a fraction of approximately 1.1% 13CO2. Similar relationships hold for other gases such as CO, CH4 and others. The isotope ratio is sufficiently constant for most technical processes, with the result that it is possible, for example, to measure 13CO2 instead of 12CO2. If there is thus a change in the composition of CO2, it is a proportional, representative change in the largely constant low fraction of 13CO2, as well. However, it is important that the concentration present here is approximately 100 times lower than when CO2 overall or 12CO2 is measured. Consequently, absorption as such in the measuring cell is so low, again, that as large as possible a residual light signal reaches the detector. This means, therefore, that when the detector measures CO2 represented by 13CO12 it measures in a clearly more favourable branch of the Lambert-Beer law. A second beam path with a second cell for a second measuring component. The photometer according to the invention operates optimally for those gas components in the case of which, for example, carbon is contained as chemical ligand in the molecules. It is then therefore possible the way according to the invention of the representative measurement of 13CO2 as a representative for CO2 also to be applied generally to other molecules such as, for example, CO or CH4 and others. In this way, the transmittance ratios are then selected such that the corresponding isotope absorption bands are shifted with respect to those of the basic element. Only thus is it possible to implement this mode of procedure in general. Thus, for example, in a gas mixture of X and Y the smaller fraction X, for example, would mean that the detector measures directly, that is to say not in an isotope-selective fashion, and the detector connected downstream thereof would be filled with Y* and thus measure the isotope relating to Y as a representative relating to the Y concentration. The only important point in this case is that the first detector should be transparent with respect to the Y* band in this frequency range; that is to say the absorption band of X is not permitted to coincide with that of Y* . This can also be extended to gas mixtures consisting, for example, of X, Y, Z and W, in which the detector X measures a basic concentration of X, again not in an isotope-selective fashion, and then three, series-connected, further detectors are arranged, of which one Y*, one Z* and the last is filled with W*. Here again, the only important point in this case is that the detectors respectively connected upstream are transparent with respect to the downstream detectors and their absorption bands, that is to say do not mutually overlap. The extinction produced upon passage through the detector window and upon passage through the gas section overall can be predetermined in this case very effectively, for example with the aid of calibration cells which are to be inserted and can be used to determine the individual extinction values.
In the embodiment according to the invention in accordance with Claim 2, the aim is specifically to measure the isotope ratio of 13CO2 and 12CO2, with the object of using this configuration according to the invention to render it possible to measure in a modal fashion. This simultaneously counters the problem, already described above, that two measuring beam paths cannot be adjusted to pure online measurement. This is achieved by virtue of the fact that only one measuring cell is used for measuring both the 13CO2 and the 12CO2. A filter cell filled with 12CO2 is located in a way according to the invention in series with the measuring cell. Arranged downstream of said filter cell is a first detector, filled with 12CO2, for measuring 12CO2, and arranged downstream thereof, is, again, a second detector, filled with 13CO2, for measuring 13CO2. No additional filtering is undertaken between the two detectors, E1 and E2.
As already mentioned above, in order to reduce the sagging of the detector characteristic the filter cell is introduced upstream of the first detector, which is filled with 12CO2. Said filter cell is filled with 12CO2, and attenuates the dominant 12CO2 main bands to such an extent that it is possible to work with the downstream 12CO2 detector in a flatter and thus more favourable region of the characteristic. The filter cell simultaneously reduces the cross-sensitivity of 12CO2 to the 13CO2 channel. Only one calibration cell is used for calibration purposes, being filled with a mixture of 12CO2 and 13CO2 and being capable of being swivelled in between the filter cell and the first detector.
A further, and thus third embodiment of the invention, as specified in Claim 3, firstly uses a design as in the second embodiment, but in this case the detectors are exchanged and there is arranged between the detectors an interference filter which passes only the overtone bands of the gas component active in the downstream detector. The arrangement of a filter cell as in the second exemplary embodiment is not mandatory, however, because 12CO2 is not measured until the last detector downstream, and only the overtone bands are still used for this purpose, after all. These overtone bands are left with just one absorption, which is approximately 100 times weaker than that of the main bands. Consequently, this is situated far in the linear region with regard to the detector characteristic.